I put the intro in the intro section.  My son and his friend dragged home a '75 DT125 that had set outside for many years.  They started pulling it apart (piston was froze and cylinder pitted).  It appeared something in the crank was also hitting a stop so was in the process of taking it the rest of the way apart.

Hit a stall with this armature removal while I ordered some 10x1.25 threaded bolts.  That did the trick.  And sitting with penetrating oil for a few more days (and the impact wrench removing the armature) seems to have loosened up the crank so it now turns over!  Decided we needed to pull it the rest of the way apart anyway to see what it looks like before going back together.  Clutch discs in the pics.  And now need to get the clutch housing off (not sure the proper name).  Requires another 'special' tool so will be making that next.

 

I see I put this in the wrong section, maybe a mod can move it over.

ADVICE WANTED:

I did get the clutch housing off and am now able to rotate the crank freely.  The various gears on the transmission side look pretty decent.  Everything is smooth turning and a closeup of the journal on the clutch housing looks decent by my untrained eyes/feel.

The crank turns over.  There is a slight grab, sometimes, just past TDC.  I put some diesel in the case and rotated it a bit to see if the grab went away - doesnt seem to have.  But its possible it is how I am holding on to the rod and maybe lightly scraping as it comes around.  Although if I put side pressure on the rod while turning it is not 'smooth' - not sure about side thrust on a rod journal though, they might normally float side to side.  

How much disassembly left to split the case and inspect the crank in more detail?  A lot of gears and mechanism on the transmission end still there, and my philosophy is to NOT take it apart any more than 'need' be.  I do want to have a chance of this running again someday.

So you experts - tell me - what would I look/feel for to determine the crank is useable and no need to pull it apart further?  How would I 'know'?

And/Or, what are the minimums to pull off to get it split - or is it in for a penny in for a pound.  So far with a bore of the cylinder and a new piston, this engine seems quite salvageable.  (and numbers match the frame if that matters - am thinking once we have a decent engine and the frame cleaned up, the rest is more straightforward).

 

The journal on the clutch, and general area looks ok to me

A bit of crud on the bottom of the counterbalance - likely to scrape off but I dont want to create too much loose particles floating around.

Then just past TDC where I feel the slight 'rub' - at least sometimes.  Again this is with some diesel fuel in it to help loosen it up.  What is NOT coming out is a bunch of filings or pasty junk nor rust.  The diesel turns color but thats about it.  Encouraging.

So what do I test/look for next??

If you want the bike to last and be reliable, split the cases, pull the crank and have rebuilt (new lower rod bearing and shaft bearings) Put new seals in as well. Anything but perfectly smooth rotation means there is a problem and a rebuild will remedy it.
alnarv

Good advice.  I decided to go all the way and moved forward with splitting it and as you suggest, it’s not great. Definitely needed it

Both main bearings shot and although the rod spins freely may as well do that while in here. 

bore the cylinder oversized and a new piston. New rod TBD still

so it needed pretty much, everything.  Will get some estimates to decide if worth rebuilding or parting out.  They are teenagers doing this project so do not have much $$ to put in it.